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1. Advances in Physics Theories and Applications
ISSN 2224-719X (Paper) ISSN 2225-0638 (Online)
Vol.23, 2013
www.iiste.org
Designing a 2D RZ Venture Model for Neutronic Analysis of the
Nigeria Research Reactor-1 (NIRR-1)
1.
2.
3.
4.
Salawu, A.1*, John R.White2, Balogun, G.I.3, Jonah, S.A.3, Zakari, Y.I4
Department of Physics, University of Abuja, Gwagwalada, Abuja, Nigeria
Department of Chemical and Nuclear Engineering, UMASS Lowell, MA-USA
Center for Energy Research and Training, ABU, Zaria, Nigeria
Department of Physics, Ahmadu Bello University, Zaria, Nigeria
* E-mail of the corresponding author: salawuhameed@yahoo.com*,
Abstract
A 2D RZ VENTURE model has been developed for the Nigeria Research Reactor-1 (NIRR-1) and this model
was used to perform neutronic analysis for the system using the code “VENTURE-PC”. The major homogenized
regions in the 2D VENTURE model include the active fuel region and the control region while the remaining
components in the system geometry where modelled as closely as possible. The reactor physics parameters
generated from the neutronic calculations include excess reactivity, control rod worth, shim worth, shutdown
margin. The model predictions of these parameters for NIRR-1 system were in good agreement with
experimental results as well as the results from similar calculations using different nuclear analysis tools. This
2D RZ VENTURE model gives an excellent simulation of the Nigeria Research Reactor-1 and the model will be
very helpful in the future analysis of the system, as well for developing an LEU core model for future conversion
of NIRR-1 from HEU to LEU fuelled research reactor.
Keywords: Model, Neutronics, geometry, code, Design, Reactor, Simulation, Calculations, NIRR-1, Shim,
Control, Shutdown, Physics. Peak, Density, Power, Flux, Neutron
1. Introduction
The initial model for the Nigeria Research Reactor (NIRR-1) core physics calculations was designed by the
China Institute of Atomic Energy with computer codes HAMMER and EXTERMINATOR-2 (FSAR, 2005).
These design calculations were repeated in Nigeria with WIMS/CITATION and MCNP (See references 1 and 2).
HAMMER and EXTERMINATOR are the first set of codes used to solve reactor physics problem (using
diffusion theory method) followed by WIMS and CITATION. Another set of codes that is manufactured and
used today in the United States (the originator of EXTERMINATOR, CITATION and MCNP) are the SCALE
code system and the VENTURE code. Many regulators, licensees, and research institutions around the world
have used these codes for reactor safety analysis and design (SCALE, 2011). The U.S Nuclear Regulatory
Commission (NRC) used SCALE code a lot for their licensing operations and evaluations. The successful core
conversion studies of the University of Massachusetts Lowell research reactor was done with the codes SCALE,
VENTURE and MCNP (White, 2011). Information available to us from the literature has shown that research
has never been conducted on any MNSR type reactors around the world using this version of the diffusion theory
code (i.e. VENTURE-PC). Therefore, a repeat design calculation for NIRR-1 is required with a view to validate
the SCALE and the VENTURE codes for the MNSR before designing an LEU core model (in the future) for
core conversion study of the system from HEU to LEU fuel. Since the present NIRR-1 core was designed with
diffusion theory codes, utilizing a recent version of such code to design an LEU core model for the system is
extremely important. The 2D RZ VENTURE model developed in this work for the present NIRR-1 system was
used to perform neutronic calculations for the system and a number of reactor design parameters were generated.
It gives an excellent simulation for NIRR-1 system and such model will be very helpful in the future analysis of
any MNSR type research reactors.
Methodology
A manual sketch of the basic geometry for the 2D RZ VENTURE model for NIRR-1 core arrangement, showing
the dimensions of some important components, is provided in figure 1. The active core region of this model is a
homogenized mixture of the fuel pin, the tied rods, the dummy pins and the moderating material (i.e. water). The
active region of the control rod is also a mixture of the poison material, the clad, the water and the control rod
guide tube. The non fuel region shown in the figure is a homogeneous mixture of water and Al alloy plug on
each end of the fuel pin. Note that the big arrow head shown in the figure 1 indicate the direction of the coolant
flow in and out of the reactor core. Figure 2 shows the result of the Matlab code written to reproduce this 2D RZ
VENTURE model as well as generating automatically some of the input data require for the criticality
calculation using the VENTURE code. In this VENTURE model for NIRR-1, the active poison region of the
control material and the control rod follower was broken into 26 and 18 regions respectively (see figure 2). This
26

2. Advances in Physics Theories and Applications
ISSN 2224-719X (Paper) ISSN 2225-0638 (Online)
Vol.23, 2013
www.iiste.org
was done in order to allow easy movement of the control rod for various criticality calculations. The result from
this calculation was used to determine the integral worth of the control rod. A total of 10 different regions were
created for the placement of top beryllium shims (see figure 2) and these were used to determine the reactivity
worth associated with beryllium shims addition.
Figure 1: A manual sketch of the basic geometry of the 2-D VENTURE model for NIRR-1 (Note:
dimensions in cm)
Result and Discussion
The 2D RZ VENTURE model developed in this work was used within the VENTURE code to generate a
number of k-effective data at different control rod withdrawal positions for the NIRR-1 system and these data
were used to determine the reactivity worth of the control rod. A plot of the reactivity changes as a function of
the control rod withdrawal distance for the present NIRR-1 system is shown in figure 3. These calculations using
the 4 group theory as well as the similar result using a simple 2-group method provided the total control rod
worth for NIRR-1 as shown in table 1. The observed slight difference in these results as provided in the table is
due to the differences in energy resolution between the two methods used. Since we could not model the
reactivity regulators in the 2D VENTURE model for NIRR-1, such a slight increase in the value of the control
rod worth shown in the table as compared to the experimental result of 7.0mk is actually expected.
27

3. Advances in Physics Theories and Applications
ISSN 2224-719X (Paper) ISSN 2225-0638 (Online)
Vol.23, 2013
www.iiste.org
NIRR1 2-D Model (with control in) -- RZ Material Map
0
Matl-Color Map
# 10 smt+water
20
# 9 smt+Be
# 8 cntl out
40
z-dimension (cm)
# 7 vessel
# 6 nonfuel
60
# 5 fuel
80
# 4 follower
# 3 water
100
# 2 Beryllium
120
# 1 poison
0
20
40
60
r-dimension (cm)
Figure 2: The main geometry and material layout of NIRR-1 core configuration showing control rod
at fully inserted position without top beryllium shims.
The value of 7.2mk of the total control rod worth obtained in this work from the 4g calculation is in good
agreement with the result of the similar calculations performed at the Centre for Energy Research and Training
of Ahmadu Bello University here in Nigeria (see references 1 and 2). Note that the total control rod worth of
7.61mk reported in the 2007 MCNP calculation was for the control rod travel length of about 27.0cm. Since the
actual travel length of NIRR-1 control rod is 23.0 cm, the total rod worth of about 7.1mk associated with 23cm
travel length from this MCNP calculation was used to make the comparison.
28

4. Advances in Physics Theories and Applications
ISSN 2224-719X (Paper) ISSN 2225-0638 (Online)
Vol.23, 2013
www.iiste.org
8
Reactivity: dk/k (mk) for 4g case
7
6
5
4
3
2
1
0
0
5
10
15
20
Distance withdrawn (cm) from bottom of active fuel
25
Figure 3: Reactivity versus control rod withdrawal length for NIRR-1 system.
The result from the simple 2 group calculations as shown in table 1 does not seem unreasonable as compared
with the experimental result or the results from the 4 group calculation which is more computationally intensive
and time consuming. It was included in this work in order to demonstrate that a simple 2 group calculation can
also give a good result for a nuclear system as compared to a more complicated calculation method.
Table 1: Total control rod worth: 2 group versus 4 group theory result (worth in mk).
Name of the Parameter
4 group theory
2-group theory
Control Rod worth
Shim worth (10.95cm)
Excess reactivity
7.20
19.64
5.55
7.83
16.15
5.63
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5. Advances in Physics Theories and Applications
ISSN 2224-719X (Paper) ISSN 2225-0638 (Online)
Vol.23, 2013
www.iiste.org
20
18
16
14
dk/k (mk)
12
10
8
6
4
2
0
0
2
4
6
8
Thickness of beryllium shim (cm)
10
12
Figure 4: The top beryllium shim worth curve for the present NIRR-1.
Similarly, a number of k-effective data as a function of top beryllium shim thickness using the 2 group and the 4
group calculation methods was also generated. These data was used to determine the reactivity worth of the top
beryllium shims. A plot of the shim worth curve generated from such data for the present NIRR-1 system using
the 4 group calculation method is as shown in figure 4.
Conclusion
The results from the neutronic calculation using the VENTURE computer code were in good agreement with
similar calculations using either the MCNP or the CITATION/EXTERMINATOR code. The control rod worth
as well as the top beryllium shim worth was slightly higher than the experimental result. This is exactly what
was expected since the reactivity regulators were pulled out of the reactor core in the 2D VENTURE model for
NIRR-1 system. The model developed in this work can be very helpful in future analyses of NIRR-1 system. The
result from the 2 group calculation does not seem unreasonable as compared with the experimental result. It was
included in this work in order to demonstrate that a simple 2 group reactor physics theory can also give a good
result as compared to the experimental result or a more complicated 4 group or higher group calculation method.
References
1. Balogun, G.I. (2003): Automating some analysis and design calculation of miniature neutron source
reactors at CERT (I), Annals of nuclear energy, Vol. 30, 81-92
2. Jonah, S.A., Liaw, J.R. and Matos, J.E. (2007): Monte Carlo simulation of core physics parameters of the
Nigeria Research Reactor-1 (NIRR-1), Annals of nuclear energy, , Vol. 34 953-957
3. FSAR, 2005: Safety Analysis Report for Nigeria Research Reactor-1, Centre for Energy Research and
Training ABU Zaria, Nigeria.
4. SCALE (2011): A Comprehensive Modeling and Simulation Suite for Nuclear Safety Analysis and Design,
ORNL/TM-2005/39 , Version 6.1, June 2011. Available from Radiation Safety Information Computation
Center at Oak Ridge National Laboratory as CCC-785
5. J. R. White and L. Bobek, “Startup Test Results and Model Evaluation for the HEU to LEU Conversion of
the UMass-Lowell Research Reactor,” 24th Intl. Mtg. on Reduced Enrichment for Research and Test
Reactors, San Carlos de Bariloche, Argentina (Nov. 2002).
30

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